1. Field of the Invention
The present invention relates to a semiconductor device fabrication method for forming an electrode with metallic paste, and more particularly to a semiconductor device fabrication method for simple and high-reliability electrode formation when using two or more types of metallic paste together.
2. Description of the Related Art
Though the present invention can be applied to various types of semiconductor devices, it is particularly effective for light receiving elements such as a photodiode and a solar cell. In this specification, a solar cell is taken as a specific example to describe the background of the present invention.
A silicon solar cell is the mainstream of solar cells for electric power and the process flow at its mass-production level is considerably simplified.
A conventional (known-art) semiconductor-device fabrication method is described below by referring to FIGS. 5A to 5J.
FIGS. 5A to 5J are a general solar-cell manufacturing flow.
In FIGS. 5A to 5J, symbol 1 denotes a p-type Si substrate serving as a semiconductor substrate to form an n-type diffusion layer 2 whose conducting type is reversed by thermally diffusing, for example, phosphorus (P) in FIG. 5B. Phosphorus oxychloride (POCl.sub.3) is generally used as a phosphorus diffusion source. Moreover, when there is not any device, the n-type diffusion layer 2 is formed on the entire surface of the p-type Si substrate 1. The n-type diffusion layer 2 has a sheet resistivity of approx. tens of .OMEGA.z,1 and a depth of 0.3 to 0.5 .mu.m.
The n-type diffusion layer 2, though details are omitted, is removed through etching so as to leave the n-type diffusion layer 2 only on one principal plane as shown in FIG. 5C after protecting either plane by, for example, resist and removing the resist by an organic solvent or the like.
Thereafter, aluminum diffuses into the p-type Si substrate 1 from aluminum paste as impurities and a p.sup.+ layer 4 containing high-concentration impurities is formed as shown in FIG. 5E by printing an aluminum paste electrode 3 on the plane opposite to the n-type diffusion layer 2 in FIG. 5C by, for example, the screen printing method (or roll coater method)(refer to FIG. 5D) and baking the electrode 3 in a near-infrared oven for several minutes to tens of minutes at 700.degree. to 900.degree. C.
This layer is generally referred to as a BSF (Back Surface Field) layer and contributes to the improvement of the energy conversion efficiency of a solar cell. Moreover, thereafter, it is possible to form an antireflection film on the surface of the n-type diffusion layer 2 though the illustration is omitted for simplification.
FIG. 5F shows a state of printing and drying a sliver paste electrode 5 without removing the aluminum paste electrode 3 on the back.
Moreover, FIG. 5H shows a state of removing the aluminum paste electrode 3 on the back by, for example, aqua regia and FIG. 5I shows a state of continuously printing and drying the silver paste electrode 5 on the back. These silver paste electrodes 5 on the back are made to function as wiring joints when fabricating a module constituted by connecting a plurality of solar cells in series and parallel. Finally, a solar cell is completed by printing the silver paste electrode 6 on the surface (light-receiving plane) and baking it again in any process and in FIGS. 5G and 5J. Moreover, it is possible to complete a solar cell by omitting the baking process in FIG. 5E for process simplification and performing one-time baking after the processes in FIGS. 5G and 5J.
However, a silicon solar cell manufactured as described above has the following problems on the back-electrode forming method.
For example, in the case of the method starting with FIGS. 5A to 5E and ending with FIGS. 5F and 5G, if the aluminum paste electrode 3 and the silver paste electrode 5 are alloyed in the electrode baking process, a problem occurs that solder does not attach the wiring at all due to alloying with aluminum when soldering the wiring with the silver paste electrode 5 or the wiring bond strength is small even if solder attaches the wiring.
Moreover, in the case of the method starting with FIGS. 5A to 5E and ending with FIGS. 5H and 5J, it is necessary to remove every aluminum paste electrode 3 through etching. Therefore, problems occur that processes become complex and the manufacturing cost increases.